1. Field
Subject matter disclosed herein relates generally to positioning and, more specifically, to performance measures for use in satellite positioning systems.
2. Information
The Global Positioning System (GPS), and other satellite positioning systems (SPSs), may rely on measurement of signal propagation delays of SPS signals traveling from positioning satellites to SPS receivers (e.g., ground-based navigation receivers, etc.). By measuring a signal propagation delay, an SPS receiver may compute a pseudorange measurement from the transmitting satellite to the SPS receiver. If pseudorange measurements can be obtained for an adequate number of satellites at known orbital positions relative to the earth, an SPS receiver may compute an estimated location of the receiver as part of a navigation solution. Typically, pseudorange measurements for four or more satellites are desirable to generate a solution, but in some instances measurements for fewer than four satellites may be used. In some situations, a navigation solution and/or “position fix” may be computed from information obtained from a diversity of sources including pseudorange measurements in combination with, for example, an externally obtained initial position. The accuracy of the initial position varies significantly, depending on the source of the initial position. For example, an uncertainty in an initial position may be as low as 15.0 meters for an initial position derived merely from acquisition of a signal from an IEEE std. 802.11 access point. For an initial position derived merely from knowledge of a location of current serving cell in a cellular communication network, an uncertainty in the initial position may be greater than 10.0 km.
A position fix often includes an estimated location of a mobile device and a measurement of uncertainty about or accuracy of the estimated location. For example, accuracy of individual measurements used for computing an estimated location may contribute to an overall accuracy of the computed location estimate. In the case of pseudorange measurements obtained by acquiring SPS signals transmitted by satellites, the accuracy of the pseudorange measurements may be affected by many factors including, for example, thermal noise.
While techniques for characterizing an uncertainty of a location estimate may account for accuracies of individual measurements used to compute the location estimate, such metrics do not account for situations where one or more data items used for computing a location estimate are completely erroneous (e.g., inaccurate beyond a perceived indication of uncertainty). While known or perceived conditions such as the thermal measurement noise level may be used for quantifying an uncertainty of a measurement value, it should be recognized that other conditions and/or events may skew a measurement value such that the value is inconsistent with the perceived level of accuracy. As such, a measurement of uncertainty of a computed location estimate may be misleading or otherwise overstate a usefulness of a position fix. Additional or alternative techniques for characterizing a usefulness of a computed position fix are desirable.